Water Adsorption Isotherms for Charged and Uncharged Cesium Iodide Aerosol Particles

Adsorption isotherms for water vapor on powdered cesium iodide are reported. Both macroscopic and microscopic (single-particle) samples of CsI powder from the same source were studied by two different techniques. The adsorption isotherms for the macroscopic samples were obtained using a Cahn 2000 Electrobalance, which leaves the sample uncharged during the measurement and utilizes a conventional microbalance to measure the relevant mass changes. The adsorption isotherms for the microscopic samples were obtained using a custom-made two-ring electrodynamic balance to measure the mass changes due to adsorption that occurred for single suspended charged particles of the CsI powder. Here, the relevant mass changes were determined from the changes observed in the required particle suspension voltages in the balance. The total charge on each particle studied was determined using the electron stepping technique. Based on the classification scheme of Brunauer, Emmett, and Teller, it is concluded that type III adsorption isotherms were observed for both the macroscopic and the single-particle CsI samples, which would indicate a multilayer adsorption process. The isotherms for the macroscopic and the single-particle CsI samples, while exhibiting the same basic shape, were found to vary greatly in magnitude. It is believed that this variation in magnitude is primarily due to the charge on the microscopic samples interacting attractively with the highly polarized water molecules and thus significantly increasing the multilayer adsorption of water. Some possible chemisorption was also indicated for both the charged and uncharged CsI, as evidenced by an inability to completely regenerate either the macroscopic or the microscopic samples.